1. Field of the Invention
The present invention relates to a two-sided imager for creating images on both sides of a sheet and, more particularly, to improvements in a fuser used in a two-sided imager in which an unfixed image on an image carrier is transferred to a sheet via an intermediate transfer body, and in which unfixed images formed on both sides of a sheet in turn are fixed separately by the fuser.
2. Description of the Related Art
A conventional imager of the intermediate transfer type is described, for example, Japanese Patent Laid-Open No. 323704/1993. This imager has a latent image carrier as consisting of a photoconductor (PC) drum. Developing units for various color components such as black (Bk), yellow (Y), magenta (M), and cyan (C) are mounted around the drum. An intermediate transfer body in the form of a belt, for example, is placed opposite to the latent image carrier. Whenever the latent image carrier makes one revolution, unfixed toner images of each color component formed on the latent image carrier are transferred onto the intermediate transfer body in turn (primary transfer). Then, the composite primary transfer image on the intermediate transfer body is transferred onto paper or sheet such as an OHP slip-sheet (secondary transfer), thus forming a desired image on the sheet.
With this type of machine, the composite toner image consisting of multiple toner images already transferred to the intermediate transfer body is transferred to the sheet at once. Therefore, this machine has the advantage that disturbance of the image and color misregistration can be effectively prevented during the multiple transfer without the need to take account of the thickness of the sheet, the surface characteristics, and the transportation characteristics of the sheet over the latent image carrier.
Some of imagers of this kind create images on both sides of a sheet. These two-sided imagers perform the following sequence of operations. In the duplex mode, an unfixed toner image transferred to one side of a sheet (front surface) is fixed by a fuser. Then, the sheet is reversed via a sheet return-and-reverse mechanism. Subsequently, the sheet is sent back to the secondary transfer position. A composite primary transfer image that is formed on the intermediate transfer body and is an image formed on the rear side of the sheet is secondary transferred to the other side (rear surface) of the sheet. Then, the image is fixed by the fuser.
The fuser has a pair of fixing rolls that are in contact with each other and roll over each other. One of the rolls is a heating fixing roll, while the other is a pressure fixing roll pressed against the heating roll. The sheet is passed through the nip between the fixing rolls. As a result, the unfixed toner on the sheet is fixed. To prevent so-called offset phenomenon (i.e., the toner image on the sheet transfers to the fixing rolls), an oil (e.g., a silicone oil) acting as a release agent is normally supplied to the fixing rolls by a release agent supply device. This method is generally employed.
With respect to the amount of the supplied release agent, it is possible to control the amount of the supplied release agent from the release agent supply device according to the amount of the release agent applied to the fixing rolls. However, to simplify the structure of the machine, a given amount of release agent is supplied, because this is a straightforward method.
In the both-sided imager of the intermediate transfer type of this kind, if a large-area, half-toned image is created in the simplex mode after the same image is printed or copied repeatedly (e.g., about 50 times) in the auto duplex mode, then the density of the image portion repeatedly printed in the auto duplex mode becomes lower than the other portions. This tends to produce a residual image, which may hereinafter be referred to as oil ghost. This phenomenon was considered a technical problem.
We have investigated causes of the oil ghost and arrived at the following conclusion.
It is assumed that a two-sided imager creates the same image repeatedly in the auto duplex mode. This imager has fusers including a fuser located on the side of the heating roll. Only this fuser is equipped with a release agent supply device that supplies a constant amount of release agent 303.
If an unfixed image is created on one side (front surface) of a sheet 300, as shown in FIG. 21(a), and if this sheet 300 is passed through the fuser 301, the release agent 303 adheres to the substantial whole of the other side (rear surface) of the sheet 300 because the release agent 303 is supplied also to the heating fixing roll 302 of the fuser 301.
When transfer is made to the rear surface of the sheet 300, the sheet 300 having one side already fixed is again passed through the secondary transfer position. At this time, as shown in FIG. 21(b), the release agent 303 adhering to the rear surface of the sheet 300 transfers to an intermediate transfer body 304, corresponding to a nonimage area 305, i.e., the area of the sheet 300 excluding an unfixed image (image area) 306 on the intermediate transfer body 304 for the rear surface.
Where the same image is printed or copied repeatedly, the release agent 303 is hardly supplied to an area of the intermediate transfer body 304 corresponding to the image area 306. It follows that the release agent 303 transfers only to the area corresponding to the nonimage area 305. Consequently, the transfer efficiency in the nonimage area 305 becomes much higher than that in the image area 306.
Under this condition, if a large-area, half-toned image is created in the simplex mode as shown in FIG. 21(c), the area corresponding to the image area 306 in the auto duplex mode appears as a half-toned image 307 of a low gray level, and the area corresponding to the nonimage area 305 appears as a half-toned image 308 of a high gray level, possibly because the area corresponding to the image area 306 in the auto duplex mode is lower in transfer efficiency than the other area corresponding to the nonimage area 305 in the auto duplex mode. It appears that this gives rise to negative ghost.
Especially, where the full-color mode of the aforementioned duplex imager for creation of full-color images is selected, the sheet transported through the fuser normally takes a longer path than where the monochrome printing mode is selected. The amount of the release agent transferring to the pressure roll from the surface of the heating roll of the fuser is increased accordingly. Hence, the aforementioned oil ghost phenomenon appears more conspicuously.
In the case of a duplex imager equipped with plural image carrier areas and thus having plural intermediate transfer bodies, images in the image carrier areas on the intermediate transfer bodies are successively transferred to plural sheets. These sheets are passed through the fuser in succession.
If the plural sheets successively pass through the fuser, this fuser is required to fix the images without offset. Therefore, the amount of the supplied release agent must be increased compared with the case in which there is only one image carrier area. In consequence, the above-described oil ghost phenomenon appears more conspicuously.